Any discussion of the background art throughout the specification should in no way be considered as an admission that such art is widely known or forms part of common general knowledge in the field.
Optical systems inherently suffer loss in signal information due to various forms of optical aberration. In smaller, simpler optical devices, beams can be propagated along trajectories closely parallel to the optical axis. In these “paraxial” configurations, aberrations are small and can generally be ignored in practice. However, as more complex devices are built to perform advanced functions, the need to propagate beams off-axis and outside the paraxial region is becoming increasingly important. In these “higher order optics” situations, a number of monochromatic optical aberrations become more distinct. In particular, off-axis curvature of the focal plane of optical elements becomes a concern. So too does spherical aberration and optical coma.
The degree of the aberrations is generally related to the size and profile of the optical beams in the system. In wavelength selective switch (WSS) devices it is often advantageous to reshape the beam profile to be highly asymmetric. For example, in liquid crystal on silicon (LCOS) based switches, elongated beam profiles are advantageous for efficiently switching many wavelength channels simultaneously. Larger and more asymmetric beams generally experience higher aberrations than smaller symmetric beams.
The asymmetric nature of certain WSS designs means that the beam spot incident onto the switching engine (LCOS, MEMs mirrors etc.) can have significant aberrations, including optical coma. As the push for smaller beam spots to achieve sharper channels increases, these aberrations limit the potential of these off-axis systems.
In spectrometer-type optical systems, aberration correction is often made by moving to a two mirror Czerny-Turner system. This system gives equal and opposite aberrations for each mirror reflection, allowing a symmetric Gaussian spot to be focused at the image plane. This type of solution is possible in a WSS system, but it comes with drawbacks: the second mirror adds cost to the system; the design creates a larger spatial footprint for the optical setup; and the optical alignment procedure is more complex. These all move against the design goals for new WSS products.